Water shortages have become a serious problem in arid regions of the world and in regions of high population density. Accordingly, there is a need for desalination technology that removes salt from seawater.
Membrane processing methods are a known desalination technology. In membrane processing methods, semi-permeable membranes are generally employed. Semi-permeable membranes are known as membranes that pass only molecules and ions of a specific size or smaller. For example, they are membranes that pass the water but not the salt in seawater. When two solutions of differing solute concentrations are brought into contact through a semi-permeable membrane, osmotic pressure is generated between the two solutions. The solvent of the solution on the side of low solute concentration, that is, the side of low osmotic pressure, passes to the side of high solute concentration, that is, the side of high osmotic pressure. This phenomenon of osmosis theoretically continues until the stage where the osmotic pressure differential decreases to zero. For example, when seawater and water are brought into contact through a semi-permeable membrane, the water tends to pass through to the seawater side and create a state of equilibrium.
The reverse osmosis process (RO process) and forward osmosis process (FO process) are known membrane processing methods utilizing such semi-permeable membranes.
The RO process is an osmotic technique whereby low-molecular-weight components such as water are caused to move from the side of high osmotic pressure back through to the side of low osmotic pressure. A high pressure exceeding the osmotic pressure differential of the two solutions is applied to the side of high osmotic pressure in the RO process to bring about such reverse osmosis. For example, when separating water from seawater, seawater and water are brought into contact through a semi-permeable membrane, and pressure exceeding the osmotic pressure differential between the seawater and the water, normally pressure greatly exceeding this osmotic pressure differential, is applied to the seawater side to cause the water in the seawater to pass through to the water side.
By contrast, the FO process is a process whereby a solution with a high osmotic pressure draw (draw solution) is employed to artificially generate an osmotic pressure differential between the two solutions and cause the water to migrate, as disclosed in Patent References 1 and 2. Specifically, a starting solution in the form of a feed solution (feed solution) and a draw solution of higher osmotic pressure than the feed solution are brought into contact through a semi-permeable membrane. When that is done, the osmotic pressure differential between the two solutions causes the water in the feed solution to pass through to the draw solution side. Subsequently, the solute component in the draw solution is volatilized and recovered to separate out the water in the feed solution. There are also cases where the concentrated feed solution is separated out.
An example of a case of separating water from seawater will be described next based on FIG. 1.
FIG. 1 shows an example of a seawater processing apparatus utilizing the FO process. The solid-line arrows show the flow of seawater or water 11 separated from seawater, and the dotted lines show the flow of draw solution or draw solution solute 12, respectively. Initially, seawater 11 and draw solution 12 come into contact through a semi-permeable membrane 13. The water in seawater 11 passes through semi-permeable membrane 13 to the draw solution 12 side. Then, in stripping column 14, the solute component of the draw solution is volatilized from the draw solution that has been diluted by the water of the seawater, thereby separating out the water 16 and solute component 15 of the draw solution. The solute component 15 of the draw solution is dissolved in draw solution that has been diluted in a gas absorber 17, and reused as draw solution 12. Numeral 18 denotes a pressure gauge.
Gasifying and separating the solute component of the draw solution in stripping column 14 requires high volatility. The solute component of the draw solution must also have a high degree of solubility so that it will dissolve in the diluted draw solution. Further, the solute component of the draw solution must naturally not pass through the semi-permeable membrane. When these requirements are not met, there will be problems in the FO apparatus or FO process in that the rate of permeation through the semi-permeable membrane by water from the feed solution will be poor; the solute in the draw solution will end up leaking through the semi-permeable membrane and migrating to the feed solution side, speeding up the rate; and the quantity of draw solution solute (stripping performance) remaining in the water obtained by volatilizing the solute component from the draw solution that has been diluted with water from the feed solution will be large.
Examples employing draw solutions in the form of solutions of ammonia ions and carbon dioxide, ammonia ion solutions, sulfur dioxide solutions, and the like are known (Patent References 2 and 3). However, none fully combining the above high volatility, high solubility, and property of not passing through semi-permeable membranes has yet been obtained.